An epitaxial growth technique is a technique of obtaining a single crystal thin film layer by vapor deposition, the single crystal thin film layer used in production of an integrated circuit such as a MOSLSI or a bipolar transistor, and is a highly important technique because the technique makes it possible to grow a uniform single crystal thin film on a clean semiconductor single crystal substrate according to a crystal orientation of the substrate and form a steep impurity gradient at a junction where there is a great difference in dopant concentration.
As a typical apparatus for performing such epitaxial growth, there are three types of apparatuses: a vertical (pancake) type, a barrel (cylinder) type, and a horizontal type. These growth apparatuses have common basic principles. The growth apparatus includes a reaction chamber inside which a susceptor is provided, the susceptor on which a single crystal substrate is mounted, a heating unit provided outside the reaction chamber and made up of a halogen lamp or the like, and other components. Of the vertical types, a apparatus processing substrates one at a time is called a single wafer processing epitaxial growth apparatus.
Here, the single wafer processing epitaxial growth apparatus will be explained with reference to FIG. 8. FIG. 8 is a schematic diagram showing an example of a conventionally-used, common single wafer processing epitaxial growth apparatus.
A single wafer processing epitaxial growth apparatus 101 has a reaction chamber 103 inside which a single crystal substrate 102 is placed, the single crystal substrate 102 with a front surface on which an epitaxial deposited is deposited, and is provided with a gas feed port 104 for introducing raw material gas/carrier gas into the reaction chamber 103 and a gas exhaust port 105 for exhausting the gas therefrom. Moreover, inside the reaction chamber 103, a susceptor 106 on which the single crystal substrate 102 is mounted is provided. Incidentally, an upper wall 107 of the reaction chamber 103 is made of silica glass.
Also, at least, in the outside of the reaction chamber 103, a heating unit 108, such as a halogen lamp, which heats the single crystal substrate 102 is provided.
When an epitaxial layer is formed on the single crystal substrate 102 by using this single wafer processing epitaxial growth apparatus 101, the single crystal substrate 102 is placed on a pocket formed in the susceptor 106, and the single crystal substrate 102 is heated to a predetermined temperature by the heating unit 108 while rotating the single crystal substrate 102 by a support shaft 109 supporting the susceptor 106 and an unillustrated rotation mechanism rotating the support shaft 109 (making the support shaft 109 rotate). Then, when a silicon single crystal layer, for example, is epitaxially grown, the growth is performed by feeding raw material gas, such as trichlorosilane, which is diluted with carrier gas such as hydrogen into the reaction chamber 103 through the gas feed port 104 at a predetermined flow rate for a predetermined time.
However, when epitaxial growth is performed by using such an epitaxial growth apparatus 101, the film thickness of an epitaxial layer deposited on a single crystal becomes nonuniform, leading to a problem with a film thickness shape.
The reason is considered to be as follows. The raw material gas introduced into the reaction chamber 103 through the gas feed port 104 is gradually consumed for the formation of the epitaxial layer when passing over the single crystal substrate 102, resulting in decreased concentration of the raw material gas in a direction from the gas feed port 104 toward the gas exhaust port 105.
On the other hand, Japanese Translation of PCT International Application Publication No. 2001-512901 discloses a method for performing epitaxial growth by using an epitaxial growth apparatus 101′ in which an upper wall 107′ of a reaction chamber is not flat but has a downward convexity, unlike the epitaxial growth apparatus 101 of FIG. 8. In FIG. 9, an example of this single wafer processing epitaxial growth apparatus 101′ is shown.
There has been an attempt to make the film thickness of an epitaxial layer uniform by, as in the epitaxial growth apparatus 101′ described above, promoting an epitaxial reaction effectively by narrowing the space in the center of the reaction chamber by placing a single crystal substrate inside the reaction chamber whose upper wall 107′ has a downward convexity and performing epitaxial growth.
However, even when such an epitaxial growth apparatus 101′ is used, an excellent film thickness shape (film thickness distribution) may not be obtained, and this growth apparatus is inadequate.
Incidentally, the flow rate of carrier gas has the greatest influence on the film thickness distribution, and it is necessary to set an optimum carrier gas flow rate for each growth apparatus.
Furthermore, as described above, it is known that the film thickness shape is also influenced by the upper wall of the reaction chamber. In addition, there is a considerable individual difference among the upper walls of the reaction chambers, and the upper walls have various shapes microscopically. Therefore, even when apparatuses of the same model are used, due to the individual difference among the upper walls of the reaction chambers, the growth apparatuses have different optimum carrier gas flow rates for the film thickness shape.
Moreover, as for the relationship between the quality etc. of the epitaxial wafer other than the film thickness shape and the flow rate of carrier gas, when priority is given to the quality of, for example, a back surface halo or back surface nanotopology, it is necessary to increase the flow rate of the carrier gas (for example, hydrogen) which is introduced into the reaction chamber. This makes the film thickness of the epitaxial layer near the outer edge of the single crystal substrate particularly small (results in the generation of the peripheral sag), lowering the degree of flatness. On the other hand, in order to increase productivity, it is necessary to decrease the carrier gas flow rate and thereby reduce by-products that would accumulate in the reaction chamber. However, in this case, it is known that the film thickness shape has a thick peripheral part, also lowering the degree of flatness.
The above-described high or low quality of the epitaxial wafer other than the film thickness shape and productivity do not necessarily match the high or low quality of the film thickness shape of the epitaxial layer, and it is difficult to strike a balance between them in an optimum state.